Electrode catalysts for fuel cells that have hitherto been used are composed of an electroconductive carbon material, such as carbon black, as a carrier and a noble metal catalyst of various kinds typified by platinum supported on the carrier. It is known that an electrode catalyst of this type involves a problem that the carbon undergoes oxidative corrosion due to potential changes during operation of the fuel cell to eventually cause the supported metal catalyst to agglomerate or fall off. As a result, the performance of the fuel cell deteriorates with the operation time. The problem has been dealt with by depositing a larger quantity of the noble metal catalyst onto the carrier than is actually needed. This method, however, cannot be regarded economically advantageous.
Then, various studies have been conducted on electrode catalysts aiming at improving the performance and economical efficiency of polymer electrolyte fuel cells. For example, it has been proposed to use an electroconductive oxide carrier that is a non-carbonaceous material in place of a conventionally employed electroconductive carbon. Among the so far proposed inorganic oxide carriers is electroconductive tin oxide.
One of the performance properties required of a catalyst carrier in view of the improvement of fuel cell characteristics is to be stable in a working environment and have low electrical resistance. The electroconductive tin oxide, while stable in a working environment, cannot be said to have sufficiently low resistance. As an approach to reducing resistance, various techniques for doping tin oxide with antimony have been studied. For example, Patent Literature 1 below describes a proton conducting inorganic oxide composed of an Sb-doped SnO2 carrier having chemically bonded on the surface thereof a particulate oxide phase containing an element selected from the group consisting of W, Mo, Cr, V, and B. Patent Literature 2 below also describes a catalyst carrier comprising Sb-doped SnO2.
In connection with the technique for doping tin oxide with a dopant element except antimony, using tantalum-containing tin oxide as a catalyst carrier for fuel cells has been proposed (see Patent Literature 3 below).